Double metal cyanide (DMC) complexes are well-known catalysts for epoxide polymerization. These active catalysts give polyether polyols that have low unsaturation compared with similar polyols made using basic (KOH) catalysis. The catalysts can be used to make many polymer products, including polyether, polyester, and polyetherester polyols. The polyols can be used in polyurethane coatings, elastomers, sealants, foams, and adhesives.
DMC catalysts are usually made by reacting aqueous solutions of metal salts and metal cyanide salts to form a precipitate of the DMC compound. A low molecular weight complexing agent, typically an ether or an alcohol, is included in the catalyst preparation. The complexing agent is needed for favorable catalyst activity.
In one common preparation, aqueous solutions of zinc chloride (excess) and potassium hexacyanocobaltate are mixed together, and dimethoxyethane (glyme) is added to the resulting slurry. After filtration and washing of the catalyst with aqueous glyme, an active catalyst is obtained that has the formula: EQU Zn.sub.3 [Co(CN).sub.6 ].sub.2 .multidot.xZnCl.sub.2 .multidot.yH.sub.2 O.multidot.zGlyme
Other known complexing agents include alcohols, ketones, esters, amides, ureas, and the like. (See, for example, U.S. Pat. Nos. 3,427,256, 3,427,334, 3,278,459, and Japanese Pat. Appl. Kokai Nos. 4-145123, 3-281529, and 3-149222). The catalyst made with glyme has been the most widely used. The complexing agents used are typically water-soluble, and have molecular weights less than 500. See, for example, U.S. Pat. No. 4,477,589 (column 3), U.S. Pat. No. 3,829,505 (column 12), and U.S. Pat. No. 5,158,922 (column 6).
Although low molecular weight polyethers such as, for example, glyme, diglyme, triglyme, and ethylene glycol monomethyl ether, have been used as complexing agents in DMC catalysts, no one has taught solid catalysts that include 5-80 wt. % of a polyether having a molecular weight greater than 500. U.S. Pat. No. 4,477,589 teaches a catalyst preparation method in which a DMC catalyst suspension in water is combined with a large proportion of a 300 molecular weight propoxylated glycerol. Volatile materials (water, glyme) are stripped from this mixture, leaving a suspension of 3.7 wt. % of the DMC catalyst in the propoxylated glycerol. The catalyst/propoxylated glycerol suspension is then used to make a higher molecular weight polyether polyol.
Double metal cyanide catalysts generally have good activity for epoxide polymerizations, often much greater than conventional basic catalysts (such as KOH). However, because the DMC catalysts are rather expensive, catalysts with improved activity are desirable because reduced catalyst levels could be used.
Regardless of whether KOH or a DMC catalyst is used to make a polyether polyol, a catalyst removal step is normally required. When KOH is used to make polyols, the crude product is typically treated with an adsorbent such as magnesium silicate, is water-washed, or is ion-exchanged to remove potassium ion residues in the polyol. Double metal cyanide catalysts are often more troublesome to remove from polyols, and many catalyst-removal methods, most involving some kind of chemical treatment, have been developed for these catalysts. Some of these methods are described in U.S. Pat. Nos. 4,355,188, 4,877,906, and 5,248,833. A preferred DMC catalyst would be easily removed from polyol products by ordinary filtration and would not require chemical treatment.
Any catalyst removal process is costly. The process steps are time-consuming, labor-intensive, and require treatment materials. Utility costs are often high. Polyol treatment generates waste products: wastewater from water washing, adsorbent filter cakes, spent ion-exchange resins.
An ideal catalyst would give polyether polyols with low unsaturation and would be active enough to allow its use at a very low concentrations, preferably at concentrations low enough to overcome any need to remove the catalyst from the polyol. An epoxide polymerization catalyst that eliminates the need for a catalyst removal step for many end-use applications would clearly be valuable to polyether polyol manufacturers.